Planning, Ordering, and Installing FRP Trail Bridges (continued) Inspecting and Maintaining FRP Bridges

Many types of inspections can be used when rating the condition of FRP pedestrian bridges. This section describes nondestructive testing (NDT) methods, required equipment, and general procedures for conducting the inspections. The NDT methods are listed in order of increasing complexity. The last six require specialized experience or equipment and should be performed by consultants under contract. This information was gathered as part of a study by the Construction Technology Laboratories for inspection of FRP bridge decks (National Cooperative Highway Research Program, Project 10-64, Field Inspection of In-Service FRP Bridge Decks). Inspections are required at least every 5 years for Forest Service trail bridges.

Most routine FRP bridge inspections use the two primary methods of visual and tap testing. More complex methods should be adopted only if the primary methods are not adequate to observe or assess unusual conditions. The cost to inspect a bridge using some of the more complex methods may be more than the cost of replacing the bridge.

Visual Testing

Visual testing (VT) is the primary NDT inspection method adopted by bridge inspectors, and is well suited for assessing the condition of FRP pedestrian bridges. The basic tools required are a flashlight, measuring tape, straightedge, markers, binoculars, magnifying glass, inspection mirrors, feeler gauges, and a geologist's pick. Visual inspection generally detects only surface defects, such as cracking, scratches, discoloration, wrinkling, fiber exposure, voids, and blistering.

To help detect defects or cracking that might go unnoticed with VT, a static or dynamic live load test can be done. Loading the bridge with an all-terrain-vehicle or any live load can help reveal hidden cracks and undesirable movement.

Tap Testing

Tap testing is the second most common type of NDT performed on an FRP bridge. Tap testing is a fast, inexpensive, and effective method for inspecting composites for delamination or debonding. The mechanics of the test are analogous to "chain drag" delamination surveys used to inspect reinforced concrete bridge decks, or for inspections of wood timbers by sounding with a hammer.

The inspector taps the surface with a hammer or coin and listens for a distinctive change in frequency, indicating a void or delamination. A clear, sharp ringing indicates a well bonded structure, whereas a dull sound indicates a delamination or void. Geometric changes within the structure also can produce a change in frequency that may be interpreted erroneously as a defect. The inspector must be familiar with the features of the structure. Tap testing does not require NDT certification. A bridge engineer or inspector can perform this NDT method with very little training.

Thermal Testing

Thermography is effective for identifying discontinuities close to the surface, such as delamination, debonding, impact damage, moisture, and voids. Thermography uses an ambient or artificial heat source and a heat-sensing device, such as an infrared (IR) camera, to measure the temperature variation within the sample. Heat can be applied to the surface by natural sunlight or by a pulsed light source. An IR camera measures the temperature variation of the object. Subsurface variations such as discontinuities or voids in the material will cause slight changes in the wavelength of IR energy that radiates from the object's surface. These discontinuities in the material or emissivity differences can be detected by IR cameras.

Acoustic Testing

Acoustic testing relies on changes in sound waves to reveal defects under loading. A structure under certain load levels produces acoustic sound (known as an acoustic emission), usually between 20 kilohertz and 1 megahertz. The emission is from the stress waves generated because of deformation, crack initiation and growth, crack opening and closure, fiber breakage, or delamination. The waves come through the solid material to the surface, where they can be recorded by one or more sensors or transducers. Acoustic tests involve listening for emissions from active defects and are very sensitive when a structure is loaded.

Ultrasonic Testing

Ultrasonic testing uses high-frequency sound in the range of 20 kilohertz to 25 megahertz to evaluate the internal condition of the material. This method involves applying a couplant (typically water, oil, or gel) to the area to be inspected and scanning the area with a transducer (or probe) attached to the ultrasonic testing machine. The couplant serves as a uniform medium between the surface of the area being scanned and the transducer to ensure the transmission of sound waves. Discontinuities that can be detected include delamination, debonding, resin variations, broken fibers, impact damage, moisture, cracks, voids, and subsurface defects. Unlike visual inspection, tap testing, or thermography, ultrasonic testing requires a high level of expertise to conduct the test properly and to interpret the data.

Radiography

Radiography uses a penetrating radiation source, such as X-rays or gamma rays, and radiographic film to capture images of defects. Differential absorption of the penetrating radiation by the object will produce clearly discernible differences on radiographic film. Radiography requires access to both sides of the structure, with the radiation source placed on one side and the film on the other. Typical discontinuities that can be detected include some delaminations and some debonds (depending on their orientation), voids, resin variations, broken fibers, impact damage, and cracks. Radiography equipment can be hazardous if not handled or stored properly. This method requires a high level of skill to conduct the test and to evaluate the images.

Modal Analysis

Modal analysis evaluates a structure's condition based on changes in the structure's dynamic response. The structure is instrumented with an array of accelerometers and dynamic load tests are performed to extract modal parameters with selected frequencies and mode shapes. This method requires capital investment for sensors and data acquisition equipment, staff training, and a relatively high skill level to set up the equipment and to reduce and interpret the data. This method should be used only if other techniques are unable to address concerns about hidden damage and the overall structural performance of an FRP bridge.

Load Testing

During load testing, a bridge is instrumented with sensors such as strain gauges, accelerometers, and displacement sensors before being subjected to a known live load with a specific loading pattern. The instruments can measure the response of the structure during load tests and help determine the bridge's long-term structural health. Load testing requires investing in sensors and data acquisition equipment, and the development of the skills needed to set up the equipment and to reduce and interpret the data. This method is used only if other methods are unable to address concerns about hidden damage and the overall structural performance of an FRP Bridge.

Comparison of Inspection Methods

Visual testing is the simplest and most commonly used method. It allows the inspector to rapidly detect gross imperfections or defects such as cracks, delamination, or damage from impacts. Visual testing often can help detect imperfections, such as lack of adhesive, edge voids, discoloration, and deformation. To a trained inspector, visual testing immediately identifies areas needing more detailed examination. This technique requires interpretation, so inspectors should be trained to know what they are looking for and what any variation might mean to the strength and reliability of the bridge component. Visual testing cannot:

• Quantify the extent of damage
  
  
• Inspect components that are not visible

Tap testing or sounding is another excellent and easy-to-use method for inspecting FRP materials for delamination. The inspector listens for any change in sounds while tapping FRP surfaces. Although tap testing can be used on pultruded sections, it is less effective in detecting delaminations or debonds. Most common problems on FRP bridges can be identified using a combination of tap testing and visual testing.

Neither tap testing nor visual testing requires specialized equipment. With some training, both methods are easy to incorporate into an inspection program. Other testing methods such as thermal testing, acoustic testing, ultrasonic testing, radiography, modal analysis, and load testing are much more complex, expensive, and time-consuming.

Qualifications for Inspectors

The Forest Service inspector and team leader qualifications in the Forest Service Manual, section 7736.3, Qualification of Personnel for Road Bridges, should be used. FRP pedestrian bridges are considered complex trail bridges. Inspectors also should have additional qualifications and experience so they can identify the need for advanced inspection methods, such as acoustic, ultrasonic, or radiographic testing, and interpret the test results. Specialized NDT engineers, employed by consultants, may need to perform these inspections.

Visual Signs of Damage and Defects

Inspectors need to look at the structure as a whole as well as at specific spots. Particular problems to look for are discussed below.

Side Trusses

All trusses should be vertical and should not have any buckling (figure 27) or out-of-plane bowing (figure 28). Either condition would be an indication of a buckling failure. The nature of FRP materials will cause such problems to become worse over time. Buckling is a particular concern if the structure will be subjected to long-term loads such as snow loads.

Figure 27—This FRP bridge in Redwood National Park began to fail
when a loaded mule train was halfway across. No one was injured.

Figure 28—The top chord bowed on the left
side truss of the Staircase Rapids Trail
Bridge in the Olympic National Forest.

Deflection

Trusses are typically designed with a slight arch that should be visible. If the arch is not present, the plans should be reviewed and compared to the structure to see if the deflection is within design specifications. Excessive deflection could be an indication of loose bolts or connection failure. The deflection should be noted and monitored closely.

Connections

All connections should be inspected carefully for cracking (figure 29). This is especially significant for connections secured with a single bolt. A two-bolt connection allows the second bolt to take up some of the load of a ruptured connection. All bolts are load bearing, so any loose connections must be tightened. Overtightening bolts may crack the FRP member, affecting its strength and structural stability.

Figure 29—This joint at the top of a vertical post was damaged
when bolts were overtightened. The material was thinner
than the ¼ inch minimum now recommended.

Blistering

Blistering appears as surface bubbles on exposed laminated or gel-coated surfaces. In the marine industry, blisters generally are attributed to osmosis of moisture into the laminate that causes the layers to delaminate, forming bubbles. FRP bridge members are not as thin as boat hulls. Osmosis to a degree that would cause blistering is rare. Trapped moisture subjected to freeze-thaw cycles might cause blistering, but the blistering probably would affect just the outside layer of the material without affecting the material's structural performance.

Voids and Delaminations

Voids are gaps within the member. They can't be seen if the composite laminate resin is pigmented or if the surface has been painted or gel coated. If the void is large enough and continues to grow, it may appear as a crack on the surface. Often, voids are hidden and can lead to delamination over time. End sections of FRP materials can delaminate during construction if connections are overtightened, causing the laminations to separate (see figure 29).

Discoloration

Discoloration of the FRP material (figure 30) can be caused by a number of factors, including:

• Chemical reactions, surface deterioration because of prolonged exposure to ultraviolet light or exposure to intense heat or fire.
  
  
• Crazing and whitening from excessive strain, visible mainly on clear resins.
  
  
• Subsurface voids that can be seen in clear resins because the material was not completely saturated with resin during manufacture.
  
  
• Moisture that penetrates uncoated exposed resin, causing freeze-thaw damage called fiber bloom.
  
  
• Changes in pigmentation by the manufacturer, although this is not a structural problem.

Figure 30—The lower section of this member of an FRP
bridge is discolored because the coating that protected
it from ultraviolet light wore off.

Wrinkling

Fabric usually wrinkles because of excessive stretching or shearing during wet out. Wrinkling is not a structural problem unless it interferes with the proper surface contact at the connection or prevents the surface veil from bonding to the internal material.

Fiber Exposure

Fiber may be exposed because of damage during transportation or construction (figure 31). Left unattended, the fibers would be susceptible to moisture and contamination, leading to fiber bloom.

Figure 31—This truss was damaged by
dragging or improper handling.

Cracks

The face of an FRP member may be cracked because connections were overtightened (see figure 29) or the members were damaged by overloading (figure 32) or impact. Cracks caused by impact from vehicles, debris, or stones typically damage at least one complete layer of the laminated material.

Figure 32—The bottom chord was damaged by dynamic loads from
ATV traffic, by bolts that were overtightened, or by overloading.

Scratches

Surface veils can be abraded from improper handling during transportation, storage, or construction. Scratches are shallow grooves on the FRP surfaces. These are usually just unsightly surface blemishes, but, if severe, they can develop into full-depth cracks. Scratches (see figure 31) are judged severe when they penetrate to the reinforcing fibers, where they can cause structural damage.

Repair and Maintenance

Damage found during inspections should be repaired. Evaluate the damage and contact the FRP manufacturer to discuss proper repair options. Some of the FRP manufacturers have developed repair manuals. Strongwell has published a Fabrication and Repair Manual that covers minor nonstructural repairs. The manual covers maintenance cleaning, sealing cuts and scratches with resin, splicing cracks, filling chipped flanges with resin, filling holes, and repairing cracks with glass material impregnated with resin.

FRP bridges need to be maintained annually to ensure that they remain in service. Cleaning decks, superstructures, and substructures helps to ensure a long life. Resealing the surface veil with resin improves resistance to ultraviolet radiation and helps prevent moisture from penetrating and causing fiber bloom. Polyurethane or epoxy paint can be applied to parts that will be exposed over the long term. If cracks, scratches, and other abrasions are not repaired, the FRP member will be susceptible to fiber bloom and deterioration.